The present invention relates to a thermoplastic polyester resin composition and molded articles thereof. More particularly, it relates to a polyester resin composition suited for producing films, containers etc. having excellent ultraviolet screening properties, and such molded articles.
Polyesters, a most typical example of which is polyethylene terephthalate, have been widely used as material for fibers, films, etc., owing to their excellent mechanical and chemical properties. Recently, attention is focused on their good adaptability to containers for carbonated beverage, fruit juice, liquid seasonings, edible oil, liquor, wine and the like because of excellent transparency, gas barrier properties and hygienic safety of polyesters. Also, use of polyester film as window-adhesive film for the purposes of energy saving, screening of heat rays and prevention of scattering of broken glass pieces as a safety measure against earthquakes is rapidly spreading in recent years as a new way of application of polyester film. In the field of agriculture and horticulture, strong request is heard for a transparent film which can selectively screen ultraviolet rays in the specific wavelength region for the purposes of promoting growth of farm products and increasing their harvests (see, for example, Japanese Patent Application Laid-Open (Kokai) No. 53-98242 (1978)).
These conventional polyester containers and films are indeed very excellent in ultraviolet screening in the short wavelength region of up to about 320 nm, but they are almost completely transmittable to ultraviolet rays in the longer wavelength region and visible light rays. When edible oil or a liquid seasoning such as sweetened cooking sake (Japanese mirin), dressing, etc., is filled in such a polyester container and kept therein for several months, the content quality tends to be degraded, for example, a delicate change tends to take place in color, taste, flavor, etc., although the degree of such degradation or change depends on the type of food contained and the keeping conditions. Such deterioration of the content is caused by the external factors such as oxygen, heat, light and especially ultraviolet rays and microorganisms. Therefore, in the case of polyester containers, since they are relatively excellent in oxygen barrier properties, it would become possible to notably prevent the quality degradation of the content even though kept in storage for a long time if the ultraviolet screening properties of polyester is further improved
On the other hand, in the case of window-adhesive film, if it allows passage of, for example, ultraviolet rays in the sunrays, they may cause discoloration or fading of various types of furniture, books and other interior household stuffs, so that it is necessary to minimize or perfectly prevent transmission of such ultraviolet rays. In agriculture and horticulture, by means of mulching with a transparent covering material capable of substantially preventing transmission of ultraviolet rays of wavelengths not greater than 370 nm, it is possible to promote growth of many useful plants and to have a high and early harvest of high-quality crops (see, for example, Japanese Patent Application Laid-Open (Kokai) No. 53-124556 (1978).
Currently, in the art, it is a common practice to add ultraviolet absorbers to polyester for said purposes. These ultraviolet absorbers, however, are generally expensive and involve difficulties in their addition. Also, these compounds usually have a high subliming tendency and most of them are poor in thermal stability, so that troubles tend to take place in the adding and molding steps. Further, in case of using a polyester film containing an ultraviolet absorber for food containers or packages, the ultraviolet absorber is liable to contaminate the content.
Various types of ultraviolet absorber (such as EP-A-263,705 and International Publication No. W086/04903) have been disclosed for solving the problems, but none of these known ultraviolet absorbers is capable of screening ultraviolet rays in the longer wavelength region.
The present invention has for its object to provide a polyester resin composition which is suitable for producing molded articles having excellent ultraviolet screening properties capable of screening ultraviolet rays even in the longer wavelength region, also has high thermal and dispersion stability and has no possibility of causing trouble such as deposition of extraneous matter on the molded articles through the molding process, and the molded articles obtained by using the polyester resin composition.
The continuous researches by the present inventors on a polyester resin composition capable of meeting the objective have led to the finding that a thermoplastic polyester resin composition comprising a thermoplastic polyester resin and a specific diimide compound(s) is capable of sufficiently screening ultraviolet rays in both short wavelength and long wavelength regions, and the present invention was attained on the basis of such finding.